From:http://www.emagictricks.co.uk/product_info.php?cPath=95&products_id=4561
Cost: a measly £7.50
Effect:The Magician places a coin (2p or 10p) under a cloth directly over a glass of liquid ( water is fine ). The spectator holds the coin, through the cloth, over the glass. On the count of three the spectator releases the coin and hears it fall into the glass. They themselves take the cloth away only to find that their coin has dissolved, the glass holds nothing more than clear liquid!
Comments:
Had a great reaction to this one. Eyes popping, staring in the glass, then picking it up peering underneath and all around it finally asking me where it had gone in a last desperate attempt at some kind of answer

No angles to worry about, can be done one to one or in front of a group, needs a certain amount of quiet at the key point, so that the coin can be heard, but that's about it.
All I can say is for £7.50 it's a no brainer. Great little trick.